The present invention relates to an optical disk device used in particular to an apparatus and method of controlling the output power of a laser on recording and reproduction of signals to and from an optical disk.
Prior art method of recording information on a disk in a recording area by using laser light includes magneto-optical recording method and phase change recording method. Both methods record information on a recording layer by focussing a laser light upon the recording layer to heat it. In these recording methods, the distribution of the temperature over the recording layer will give a large influence upon the shape of a recording mark, that is, the quality of reproduced signals. The factors, which determine the temperature distribution, include the pulse width and power of recording light, etc. The current-emitted power characteristics of the semiconductor laser depends upon the temperature. Since the laser light having a power, which is ten times larger than that on reproduction, is necessary on recording, heating of the semiconductor laser becomes larger so that the temperature of the semiconductor laser is elevated. As a result, the current-emitted power characteristics may change. Accordingly, in order to record information in a good manner, it is also necessary to control the power of emitted light of the semiconductor laser on recording.
Under such a circumstance, a method of power control on recording has been proposed as is represented by Japanese Laid-Open Patent Publication No. 09-288840.
Now, a prior art power control system, which is disclosed in the above-mentioned publication, will be described.
FIGS. 1 and 2 are a block diagram and timing chart showing the prior art power control system, respectively.
In FIG. 1, a laser light emitted from a semiconductor laser 4 is passed through a collimator lens (not shown) and a rise mirror and focussed on a recording layer of an optical disk by portion of an objective lens. A part of the laser light is split in the course of its optical path and is incident upon a photodiode 5. On reproduction, the semiconductor laser 4 is supplied with a current which is obtained by voltage/current converting by portion of a V/I converting circuit 3 an output of the adding circuit 2 which is a sum of an output of a reproduced power presetting circuit 1 and an output of an amplifying circuit 13-1 which will be described hereafter, so that the laser generates a laser light having a power depending upon the supplied current value. A current output from the photodiode 5 is converted into a voltage signal by portion of an I/V converting circuit 6, and is then input to S/H (sample and hold) circuits 21-1-3. The S/H circuit 21-1 samples/holds the output from the I/V converting circuit 6 in response to timing pulses from a timing pulse generating circuit 11 for outputting it to a subtracting circuit 12-1.
Since timing pulses each having a given period are input to the S/H circuit 21-1 from the timing pulse generating circuit 11, the output of the S/H circuit 21-1 becomes an output signal of the I/V converting circuit 6, that is, the signal which is obtained by sampling/holding the magnitude of the light emitted from the semiconductor laser 4 at intervals of a given period of time.
In a subtracting circuit 12-1, the output signal from the S/H circuit 21-1 is compared with a signal from a reference signal generating circuit 10. The difference is input to an amplifying circuit 13-1 and is amplified at a given gain and then input to an adding circuit 2.
In such a manner in a reproducing mode of operation, a closed loop (is formed by a reproduction power presetting circuit 1, adding circuit 2, V/I converting circuit 3, semiconductor laser 4, photodiode 5, I/V converting circuit 6, S/H circuit 21-1, subtracting circuit 12-1, reproducing power reference signal generating circuit 10 and the amplifying circuit 13-1. The power emitted from the semiconductor laser 4 is controlled to a predetermined power.
In recording mode of operation, a peak power ON signal and an erasing power ON signal corresponding to a recording signal and recording gate signal (/WG) are input to current switch circuits 16, 18 from a recording pulse generating circuit 15.
The current switching circuits 16, 18 conducts or blocks the current signals output from the V/I converting circuits 27-1, 27-2 in response to the peak power ON signal and the erase power ON signal to impress the semiconductor laser 4 with it. An output signal from the adding circuit 26-1 which is a sum of the output signal from the peak power presetting circuit 24 and the signal from the amplifying circuit 13-3 is input to the V/I converting circuit 27-1. An output signal from the adding circuit 26-2 which is a sum of the output signal from the erasing power presetting circuit 25 and the signal from the amplifying circuit 13-2 is input to the V/I converting circuit 27-2.
On the other hand, the semiconductor laser 4 is constantly impressed with a current which is obtained by voltage/current converting by the V/I converting circuit 3 the output from the adding circuit 2 which is a sum of outputs of the reproducing power presetting circuit 1 and the amplifying circuit 13-1. When the current switching circuit 16 is turned on, the semiconductor laser 4 is impressed with a sum current of the output currents from the V/I converting circuits 3 and 27-1. When the current switching circuit 18 is turned on, the semiconductor laser 4 is impressed with a sum current of the output currents from the V/I converting circuits 3 and 27-2. Pulse light that is shown in FIG. 2 is emitted from the semiconductor laser 4.
A part of the emitted light is input to the photodiode 5 similarly to the reproducing mode. The photodiode 5 outputs a current corresponding to the incident light quantity to the I/V converting circuit 6. The I/V converting circuit 6 converts the output current from the photodiode 5 into a voltage signal for outputting the signal to the S/H circuits 21-1 through 21-3.
Three kinds of timing pulses are output from the timing signal generating circuit 11. When the pulse emitted light from the semiconductor laser 4 is the reproducing power, a timing pulse for holding the output signal of the I/V converting circuit 6 is output to the S/H circuit 21-1. When the pulse emitted light from the semiconductor laser 4 is the erasing power, a timing pulse for holding the output signal of the I/V converting circuit 6 is output to the S/H circuit 21-2. When the pulse emitted light is the peak power, a timing pulse for holding the output signal of the I/V converting circuit 6 is output to the S/H circuit 21-3. Accordingly, the S/H circuits 21-1 through 21-3 output the output signals of the I/V converting circuit 6 corresponding to the reproducing, erasing and peak powers, respectively.
The output signals of the S/H circuits 21-1 through 21-3 is input to the subtracting circuits 12-1 through 12-3, in which they are compared with the output signals from reproducing power reference signal generating circuit 10, erasing power reference signal generating circuit 22 and peak power reference signal generating circuit 23, respectively. The differences are input to the amplifying circuits 13-1 through 13-3 as the outputs from the subtracting circuits 12-1 through 12-3, respectively.
The amplifying circuits 13-1 through 13-3 amplify the input signals at predetermined gains and output the signals to the adding circuits 2, 26-2 and 26-1.
In the prior art method, the peak, erase and reproducing (bias) powers of the pulse light for recording can be controlled to respective predetermined powers as well as the emitted light power for reproducing by the above-mentioned arrangement.
In the above-mentioned prior art method prior art method, it is necessary for the output signal of the I/V converting circuit 6 to have about the same bandwidth as that of the pulse emitted light of the semiconductor laser 4 as is apparent from FIG. 2.
The pulse width of the pulse light may be about one quarter of the bit period in the narrowest case. If data having a bit period of 40 nsec is recorded, the pulse width would then become 10 nsec at minimum and the output signal of the I/V converting circuit requires a bandwidth which is about 100 MHz.
Accordingly, the photodiode 5 and the I/V converting circuit 6 also require a bandwidth of about 100 MHz and higher. This should be achieved to reduce the cost.
Since it is necessary for the S/H circuits 21-1 to 21-3 to complete the sample/hold operation within 10 nsec or less, the circuits having a faster speed and higher accuracy are required. This should be achieved to reduce the cost.
As mentioned above, the current-output characteristics of the semiconductor laser changes with its temperature. FIG. 3 shows the current-output characteristics of the semiconductor laser 4 when its temperature changes.
Since the threshold current and differential efficiency (gradient) in the current-output characteristics of the semiconductor laser 4 changes with its temperature, the prior art contemplates to compensate for both. As is apparent from FIG. 3, the change in temperature of the differential efficiency is less than the change in temperature of the threshold current.
If the power of the emitted light is represented as Pp when the semiconductor laser 4 is impressed with a current Ip at a given temperature T1, the plot of the current-emittedpower characteristics of the semiconductor laser 4 is shift rightward as shown in FIG. 3 and the gradient changes from a rectilinear line as represented by a broken line to a solid line. Accordingly, only the emitted power of Pp-xcex94P1 is obtained with the same current.
If the semiconductor laser 4 is supplied with the current Ip plus the changes in the threshold current xcex94Ith, the emitted power would become Pp-xcex94P2, which approaches the emitted power Pp at temperature T1.
Recently, the deterioration of the quality of the recording mark with respect to the change in the peak power has been reduced by the improvement in the storage medium. It is not advantageous to compensate for the change in temperature in the differential efficiency by using an expensive circuit.
It has been found that the power of the emitted pulse light at beginning of recording is approximately equal to a preset value since the temperature of the semiconductor laser per se does not change so fast.
The present invention was made under such circumstances. It is an object of the present invention to provide an optical recording/reproducing device which is capable of recording/reproducing information at a practically good quality and which has a structure which is suitable for reduction in cost.
In order to accomplish the above-mentioned object, a first aspect of the present invention provides an optical disk device for recording signals on an optical disk which constitutes a signal recording medium by focussing laser light on the face of said optical disk comprising:
an averaged light quantity calculating portion for determining the averaged light quantity of the laser light by splitting the laser light focussed upon the face of said optical disk and by detecting its low frequency component; a memory portion for outputting the averaged light quantity in a reproducing mode of operation and for memorizing and outputting said averaged light quantity which has been calculated for a first period of time (t1) after the beginning of recording in a mode of recording operation; an error quantity calculating portion for calculating the difference between signals output from said averaged light quantity calculating portion and said memory portion; and laser power control portion for controlling the power of said laser light in response to an output signal from said error quantity calculating portion.
The optical disk device may preferably further include an error quantity holding portion for holding the output value of the signal which is output from said error quantity calculating portion immediately before the beginning of recording and the laser power control portion may be controlled in response to the output signal from said error quantity holding portion for a second period time (t2) after the beginning of recording.
The optical disk device may preferably further include a reference signal generating portion for generating a signal having a predetermined level and in lieu of the output signal from said memory portion, the output signal from said reference signal generating portion may be input to said error quantity detecting portion for a third period of time (t3) after the beginning of recording.
The averaged light quantity-calculating portion may comprise a photoelectric converting portion for converting laser light into a current or voltage signal having a response frequency which is not higher than the lowest frequency of signals to be recorded.
The memory portion may preferably comprise an A/D and a D/A converting portions.
There may be a relation t1 less than t3xe2x89xa6t2 among said first, second and third periods of time (t1), (t2) and (t3).
In order to accomplish the above-identified object, a second aspect of the present invention provides an optical disk device for recording signals on an optical disk which constitutes a signal recording medium by focussing laser light on the face of said optical disk comprising:
a monitoring portion for splitting the laser light focussed on said optical disk face and for monitoring the power of the laser light; an averaged light quantity calculating portion for calculating the averaged light quantity of the laser light which is detected by said monitoring portion; a memory portion for outputting the averaged light quantity in a reproducing mode of operation of the signals and for memorizing and outputting said averaged light quantity which has been calculated for a first period of time (t1) after the beginning of recording in a mode of recording operation; an error quantity calculating portion for calculating the difference between signals output from said averaged light quantity calculating portion and said recording portion; and a laser power control portion for controlling the power of said laser light in response to an output signal from said error quantity calculating portion.
The optical disk device may preferably further include an error quantity holding portion for holding the output value of the signal which is output from said error quantity calculating portion immediately before the beginning of recording and said laser power control portion may be controlled in response to the output signal from said error quantity holding portion for a second period time (t2) after the beginning of recording.
The optical disk device may preferably further include a reference signal generating portion for generating a signal having a predetermined level and in lieu of the output signal from said recording portion, the output signal from said reference signal generating portion may be input to said error quantity detecting portion for a third period of time (t3) after the beginning of recording.
The averaged light quantity calculating portion may comprise a low-pass filter having a cutoff frequency which is not higher than the lowest frequency of band of signals which are reproduced from said optical disk.
The memory portion may preferably comprise an A/D and a D/A converting portion.
There may be a relation t1 less than t3xe2x89xa6t2 among said first, second and third periods of time (t1), (t2) and (t3).
It is possible to combine the above-mentioned features as of the present invention as many as possible.